Investigation of the Olive Mill Solid Wastes Pellets Combustion in a Counter-Current Fixed Bed Reactor
2. Materials and Methods 1. Samples Preparation
Olive pomace (OP) and olive pits (OPi) used in this study were obtained from the Zouila Oil Press Company located in the Sahel region of Tunisia (Mahdia-Tunisia). About 6 kg of pellets were prepared from the biofuels at EIFER (European Institute for Energy Research, Karlsruhe, Germany): 100% OP with a die compaction rate of 1:5 (conical pressing) and 100% OPiwith a compaction rate of 1:4 at 24 mm press channel length for both. The pellet press used is a Kahl lab scale flat matrix press 14–175 (with a maximum olive pellet production in the range 15–20 kg/h). The specific optimum rotation frequency was determined to be 85 Hz, and the optimum temperature for stable pellets in literature was 75◦C. The produced pellets were in compliance with the German and European standards (EN 303-5, EN 17225-6). After pelletization and air-cooling, the pellets were stored for minimum 24 h under room conditions to equalize any moisture differences as shown in Table1.
Table 1.Operating pelletization condition.
Pelletizer Performance 100% Olive Pomace (OP) 100% Olive Pits (OPi)
Pelletizing temperature (◦C) 60 51
Biomass moisture (% w.b.) 14.7±0.5 15.3±0.3
Pellet moisture (% w.b.) 12.4±0.4 13.7±0.2
(% w.b. is the percentage on wet basis).
Table1indicates that the moisture content decreases after pelletization. This decrease is due to the evaporation associated with the rising temperature during the pelletization process. Finally, the obtained products were moisture-balanced cylindrical pellets of 6 mm diameter and 20–30 mm in length. As it is notable, the two OMSWs were pelletized under two different temperatures 60 and 51◦C for OP and OPi, respectively. Indeed, because of its oil content and lubricity, pelletizing of olive pomace caused higher resistance (see compaction rate), and the higher friction, subsequently, raised the temperature to evaporate more water than during olive pits pelletizing. Hence, final moisture contents should be different because pelletization started with different initial moistures under different temperatures and calculations were done on wet basis. It can be observed that the difference between final moisture content on wet basis does not exceed 1.3%. Thereby, even though the moisture directly affects the low heating value (LHV), the difference between the two fuel types will be small.
2.2. Samples Characterizations 2.2.1. Raw Samples Characterizations
Ultimate and proximate analysis of the raw samples as olive pomace (OP), olive pits (OPi) and sawdust (S) are summarized in Table2.
Tables2and3show the ultimate and proximate analyses and the energy characteristics of the used raw materials. These characteristics are compared with those found in the literature for different agro-industrial wastes. All analyses were carried out at the Chemical and Microbiological Institute UEG GmbH (Germany). Ultimate, proximate and energy contents were realized according to the standards analytical methods for solid fuels. High heating value (HHV) is measured using a calorimetric bomb, and theLHVis then calculated using:
LHV=HHV−Lv
9% H+%M 100
(1)
where,LHVis the low heating value,HHVis the high heating value, % H is the hydrogen content and
%Mis the moisture content.
The energy density (ED) is obtained when multiplying the bulk densityρBDby theLHV:
ED=ρBD×LHV (2)
where,EDis the energy density andρBDis the bulk density.
Table 2.Raw materials characteristics.
Samples Equivalent
Formula d.b.a.f.a
Ultimate Analysis (% d.b.) Proximate Analysis (% d.b.)
% C % H % O % N % S % ash % FC % VM
Olive pomace (OP) CH1.54O0.56N0.024 52.2±0.8 6.70±0.3 39.6±0.6 1.50±0.1 0.08±0.01 5.10±0.10 18.90 *±0.3 76.0±1.0 Olive pits (OPi) CH1.97O0.92N0.018 41.4±0.4 5.20±0.2 52.5±0.9 0.91±0.1 <0.2 0.80±0.1 15.20 *±0.2 84.0±0.8 Sawdust (S) CH1.49O0.6N0.0035 51.5±0.5 6.40±0.3 41.9±0.5 0.20±0.1 <0.1 0.5±0.1 24.50 *±0.10 75.0±1.0
Spruce wood [24,25] CH1.41O0.59N0.0033 51.9 6.10 40.9 0.30 0.30 1.70 18.10 80.2
Wood [26,27] CH1.46O0.6N0.0016 51.6 6.3b 41.5 0.10 0.10b 1.0 17.0 82.0
Palm Kernels [3,27] CH1.52O0.58N0.038 51.0 6.50 39.5 2.30 0.27 5.20 17.50 77.30
OP (Turkish) [28] CH1.36O0.53N0.021 51.3 5.85 36.9 1.27 0.08 4.51 17.90 71.17
OP (Italy) [29] CH1.57O0.91N0.034 44.2 5.80 48.2 1.80 - 5.40 29.60 65.0
OPi(Spain) [17] CH1.71O0.57N0.0009 52.2 7.48 40.0 0.06 <0.1 0.56 18.50 80.94
OPi(Spain) [30] CH1.6O0.82N0.0019 44.8 6.0 49.1 0.10 0.01 1.40 13.80 74.40
% d.b. is the percentage on dry basis; * Calculated by difference: % O = 100−(% H + % C + % N), % FC = 100−(%
ash + % VM),adry basis ash free,bAverage value,-Not determined.
Table 3.Energy contents in the raw materials.
Samples LHV(MJ/kg) ρBD
kg/m3
ED(GJ/m3)
Olive pomace (OP) 17.90±0.40 539±10 9.60±0.50
Olive pits (OPi) 17.29±0.20 764±12 13.20±0.36
Sawdust (S) 16.30±0.10 103±3 1.60±0.06
Spruce wood [24,25] 18.10 105 1.90
Palm Kernels [26,27] 17.00 500 8.50
OC (Turkish) [31] 19.60 591 11.58
OC (Jordan) [32] 23.056 558 12.86
OPi(Spain) [8] 14.70 ** 651.90 ** 9.85
Oke (Greece) [33] 19.36 573 11.09
Rice Husks [34] 14.90 200 2.820
OC: Olive cake, Oke: Olive kernels, ** As received.
Furthermore, Table2 shows high Nitrogen contents for OP and OPi in comparison with the sawdust (0.2%) and wood (0.1%). This fact will explain why nitrogen oxide emissions were relatively high.
As can be seen from Table2, the ash content in the two samples (OP and OPi) is high (3% and 4.7%) compared with 0.5% value for sawdust. For the two samples, the Energy density is higher than many biomass types in Table3. Hence, the pelletization process is a compulsory process to increase the energy density of pellets and to make their transport and storage easier, ensuring a high hardness and long durability [35,36].
2.2.2. Pellets Samples Characterizations
Two pellets samples types were prepared: 100% olive pomace (OP) and 100% olive pits (OPi).
It needs to be clarified that the number of pellets used depends on the nature of characterization test we realize. For example, in the case of measuring the average length, diameter, and unit density (mass of the pellet divided by its volume) at least a 100 pellets are required to decrease the error according to the central limit theorem (ε∼√1N), whereεis the error and N is the number of samples. For the bulk density determination, the volume of the container (100 cm3in our case) ensured a large number of pellets was achieved. For the moisture content determination, the volume of the stove allowed a maximum of 10 crucibles containing one 1 pellet each to be used. The same process was undertaken when determining the ash content with a muffle furnace. However, for measuring theHHVvia a calometric bomb, we used about 1 g of pellet and we repeated the test 5 times to attain the average
value (this test is delicate and costly). In addition, during the pyrolysis tests to determine the volatile matter in a thermogravimetric balance, tests were repeated 3 times because this test type is time and cost intensive. Nevertheless, it was found that relative uncertainties were smaller than 5% of the mean value.
The main chemical characteristics of the produced pellets based on ultimate and proximate analysis are summarized in Table4. These analyses are compared to the standard wood pellets and to other pellets presented in the literature. We observe that our prepared pellets show typical compositions when compared to other biomasses available in the literature [16,17,37].
The ash content was determined using a muffle furnace for which the temperature was fixed at 815◦C. The resulting values were lower than those fixed by the European normalization (<5%).
In addition, the nitrogen content was 1.26% for OP and 0.61% for OPi, respectively. These values were relatively higher than 0.11% obtained for wood pellets. The sulphur content was <0.1% for both samples. Therefore, SOxemissions are expected to be insignificant.
Table 4.Pellets characteristics.
Samples Equivalent Formula d.b.a.f.
Ultimate Analysis (% d.b.) Proximate Analysis (% d.b.)
% C % H % O % N % S % ash % FC % VM
100% OP CH1.3O0.57N0.021 49.50±0.50 5.4±0.5 43.70 *±1.80 1.3±0.6 <0.10 2.90±0.10 17.70 *±0.10 79.40±0.20 100% OPi CH1.65O0.74N0.011 46.50±0.80 6.3±0.1 46.60 *±0.90 0.5±0.1 <0.10 1.90±0.10 15.70 *±0.20 82.40±1.00 wood pellets CH1.4O0.63N0.002 46.30±0.20 5.4±0.2 48.19 *±1.00 0.11 <0.10 0.30±0.02 24.30±0.90 75.40±0.70
OP (Spain) [16] CH1.52O0.49N0.032 54.75 6.17 37.00 1.98 <0.10 5.55 17.28 77.17
OP (Spain) [17] CH1.65O0.47N0.007 58.20 6.00 35.40 0.40 0 2.50 17.69 79.81
* Calculated by difference.
In Table5theLHVvalues range between 17.45 and 20.36 MJ/kg. The energy density values of our samples were reasonably good (14.42 GJ/m3for OPiand 15.59 GJ/m3for OP). However, the effect of pelletization was much more notable with OP than with OPi. This is because of the difference in the bulk density between the two raw materials attributable to the better compressibility of the pomace (along with higher pressing energy needs) compared to the pits. On the other hand, the durability (Du), which is determined as a function of the percentage of fine particles leaving the pellets after appropriate mechanical tests, showed acceptable percentages with high regression factor (R2= 0.985) [38]. Indeed, after pelletization the pellets can be stored for longer before its use. During this period fine particles can leave the pellets so that the mass and, thereby, the energy of the fuel decreases. By using a centrifugation system based in a gyratory motor under standard normalizations (ISO 17831-1), the mechanical durability can be determined by weighing the mass before starting the experiment and subsequently. Thereafter, a percentage of material which is remaining in the fuel can be calculated: This is the so-called durability. The durability provides a reasonable assessment in regard to the transport and storage of a given solid biofuel [39]. Values of durability for both prepared samples (OP and OPi) were in the same range of standard wood pellets. In addition, obtained durability (88–89%) for the peanut hull pellets agreed with our results [38].
Table 5.Energy contents in the pellets samples.
Samples LHV(MJ/kg) ρBD kg/m3
ED(GJ/m3) Du(% w.b.)
OP pellets 19.02±0.40 820±15 15.59±0.62 88±2
OPipellets 18.38±0.10 785±10 14.42±0.25 85±2
wood pellets 17.45±0.30 660±8 11.51±0.33 89±2
OP (Spain) [16] 20.36 780 15.80 -
OP (Tunisia) [37] 19.23 920 17.69 -